APPLICATION OF RAYLEIGH WAVE INVERSION FOR VOID DETECTION USING MULTICHANNEL ANALYSIS OF SURFACE WAVE INVERSION (MASW)

Shallow subsurface investigation for void detection has been an on-going research for multiple purposes in various fields of studies such as environmental science, engineering, and archeology. In this study, we show that shear wave velocity modeling could be a useful tool for near surface characterization to detect voids using Multichannel Analysis of Surface Wave Inversion (MASW). We have acquired P-wave seismic data using 48 vertical component geophones with 1 m receiver and 3 m source spacing respectively along a 2D profile over two known targets. The first is a water pipe of ~0.8 m diameter located ~1.5 m below the surface. Second, a storm drainage pipe (SDP) of ~1 m diameter at ~2 m depth. The Geogiga Seismic Pro MASW package was applied for the modeling. At first, the acquisition geometry was merged with the data which is followed by muting, filtering, and windowing of the data for ground roll in the preprocessing stage. Then, the dispersion curve is analyzed using the f-v plot to determine the variation of velocity with frequency and establish an initial velocity model. The S-wave velocity models derived from MASW show considerable velocity variation over the anomalies for both the water pipe and storm drainage pipe. A change in shear wave velocity is expected as the shear wave velocities do not travel through liquid or air. Although it may not be clear or difficult to interpret the pipes directly, a disturbance in the soil is observed. The disturbance detected could be the result of the pipe itself, or disturbance induced in the soil during the construction. Our study suggests that anomalous shear wave velocities derived from the Rayleigh wave inversion using MASW indicate relative changes due to the disturbance in the soil because of construction with respect to the background than the actual anomaly itself.